Bright light is a powerful non-pharmacological therapy for neurochronobiological sleep disorders, including sleep phase insomnias and dyssomnias, shift work sleep disorders, jet lag and certain affective disorders. The strongest phase-shifting effects oflight may often occur circa 4-5 a.m. Therefore conventional illumination methods frequently require either awakening patients or treating them at non-optimal times. Conventional light treatment also wastes precious waking hours, making compliance a major problem. Roger J. Cole, Ph.D. has patented a special sleep mask which provides timer-controlled illumination through closed eyelids during sleep. The mask's competitive advantages include convenience, portability, precise light dosage, and low power during light mask use. Mask treatment both suppresses melatonin secretion and phase-shifts the circadian body temperature rhythm. However, a more rigorous demonstration of phase shifting efficacy is needed to proceed with commercial development. This requires improved mask prototypes. I Phase I, it is proposed to develop such prototypes, using proprietary advanced optical technology, and to demonstrate convincingly that light masks can correct a circadian phase disorder. In Phase II, the investigators will develop a commercial-quality mask/controller system, perform detailed dose-response studies and undertake clinical trials.